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这次介绍一个大家可能会用到的函数，但是又与我们常用的函数有所区别。
lengths
一看，还以为是length函数，我们要介绍的函数后面多了一个s，专门用于计算列表list的长度，下面用例子来呈现我们的函数。
首先，我们先来介绍length函数，再来介绍lengths，最后对两个差异的地方进行对比，查看区别。
length
Get or set the length of vectors (including lists) and factors, and of any other R object for which a method has been defined
帮助文档内容：x是向量，因子，列表
> length(diag(4)) # = 16 (4 x 4)
[1] 16
> length(1:5)
[1] 5
> length(options()) # 12 or more
[1] 76
> length(y ~ x1 + x2 + x3) # 3
[1] 3
> length(expression(x, {y <- x^2; y+2}, x^y)) # 3
[1] 3
> require(stats)
> fm1 <- lm(breaks ~ wool * tension, data = warpbreaks)
> length(fm1$call) # 3, lm() and two arguments.
[1] 3
> length(formula(fm1)) # 3, ~ lhs rhs
[1] 3
lengths
Get the length of each element of a list or atomic vector (is.atomic) as an integer or numeric vector.
x为列表、向量
> require(stats)
> l <- split(airquality$Ozone, airquality$Month)
> l
$`5`
[1] 41 36 12 18 NA 28 23 19 8 NA 7 16 11 14 18 14 34 6 30
[20] 11 1 11 4 32 NA NA NA 23 45 115 37
$`6`
[1] NA NA NA NA NA NA 29 NA 71 39 NA NA 23 NA NA 21 37 20 12 13 NA NA NA NA NA
[26] NA NA NA NA NA
$`7`
[1] 135 49 32 NA 64 40 77 97 97 85 NA 10 27 NA 7 48 35 61 79
[20] 63 16 NA NA 80 108 20 52 82 50 64 59
$`8`
[1] 39 9 16 78 35 66 122 89 110 NA NA 44 28 65 NA 22 59 23 31
[20] 44 21 9 NA 45 168 73 NA 76 118 84 85
$`9`
[1] 96 78 73 91 47 32 20 23 21 24 44 21 28 9 13 46 18 13 24 16 13 23 36 7 14
[26] 30 NA 14 18 20
> lengths(l,use.names = F)
[1] 31 30 31 31 30
> lengths(expression(u, v, 1+ 0:9))
[1] 1 1 3
> f <- c(rep(1, 3), rep(2, 6), 3)
> f
[1] 1 1 1 2 2 2 2 2 2 3
> dates <- split(as.POSIXlt(Sys.time() + 1:10), f)
> dates
$`1`
[1] "2016-12-09 21:09:50 CST" "2016-12-09 21:09:51 CST"
[3] "2016-12-09 21:09:52 CST"
$`2`
[1] "2016-12-09 21:09:53 CST" "2016-12-09 21:09:54 CST"
[3] "2016-12-09 21:09:55 CST" "2016-12-09 21:09:56 CST"
[5] "2016-12-09 21:09:57 CST" "2016-12-09 21:09:58 CST"
$`3`
[1] "2016-12-09 21:09:59 CST"
> lengths(dates,use.names=F)
[1] 3 6 1
lengths 和 length
> f <- c(rep(1, 3), rep(2, 6), 3)
> f
[1] 1 1 1 2 2 2 2 2 2 3
> dates <- split(as.POSIXlt(Sys.time() + 1:10), f)
> dates
$`1`
[1] "2016-12-09 21:09:50 CST" "2016-12-09 21:09:51 CST"
[3] "2016-12-09 21:09:52 CST"
$`2`
[1] "2016-12-09 21:09:53 CST" "2016-12-09 21:09:54 CST"
[3] "2016-12-09 21:09:55 CST" "2016-12-09 21:09:56 CST"
[5] "2016-12-09 21:09:57 CST" "2016-12-09 21:09:58 CST"
$`3`
[1] "2016-12-09 21:09:59 CST"
> lengths(dates,use.names=F)
[1] 3 6 1
> length(dates)
[1] 3
length得到的是list的个数，lengths是得到每个list下的个数。这个是计算时很重要。顿时感觉这个函数很神奇。哈哈哈！
